The present invention is directed to bicycle components and, more particularly, to a low-normal rear derailleur having a double servo pantograph mechanism which moves the guide pulley both axially and radially with respect to the rear sprocket cluster.
Rear derailleurs having a double servo pantograph mechanism which moves the derailleur guide pulley both axially and radially of the rear sprocket cluster are known from U.S. Pat. No. 4,610,644. The derailleur shown in FIG. 1 of that patent comprises a transformable linkage mechanism comprising a base member 2, two linkage members 3 and 4 and a movable member 5; and a chain guide 6 having a guide pulley 61 and a tension pulley 62. The linkage members 3 and 4 are slanted relative to a plane perpendicular to the axis of the rear sprocket assembly so that the guide pulley moves radially as well as axially when the transformable linkage mechanism is operated. A first spring 14 is disposed between the base member 2 and the fixing member 1, and a second spring 13 is disposed between the movable member 5 and the chain guide 6. When the transformable linkage mechanism moves the guide pulley 61 axially and radially of the rear sprocket assembly S, the first and second springs 14 and 13 balance with each other to set the positions of the base member 2 and movable member 5 to further set the radial position of the guide pulley 61 relative to the rear sprocket assembly S.
By adjusting the angle of the linkage members 3 and 4 and the tension of the springs 13 and 14, a substantially constant and short chain gap (the distance between the guide pulley 61 and the selected sprocket in the rear sprocket assembly S) can be obtained. This, in turn, allows smooth shifting of the derailleur by eliminating the requirement of overshifting the chain when shifting the chain from one sprocket to another. Overshifting refers to the phenomenon where the guide pulley must be moved beyond the destination sprocket before the chain will engage the destination sprocket. Overshifting is not desirable because it creates the risk that the chain will rasp against or derail from the destination sprocket.
A typical derailleur which employs the teachings of the above-noted patent ordinarily has a return spring which biases the transformable linkage mechanism so that the chain guide is forced towards the smallest sprocket in the rear sprocket cluster. This type of derailleur is called a high-normal derailleur. Thus, when the chain is to be shifted to a larger sprocket, which is usually required when the cyclist is ascending a hill or navigating rough terrain, the manual shift control must operate against the force of the return spring. In many cases the force of the return spring is quite high, so the cyclist must exert substantial effort to shift the derailleur. The effort is further increased when the derailleur must be shifted under load, as often occurs when the cyclist is ascending a hill. This can be quite bothersome to the cyclist when he or she is already exerting substantial effort to overcome the hill or rough terrain. Additionally, when the bicycle is being operated under a severe load with a conventional high-normal derailleur, the chain sometimes skips around the destination sprocket and fails to properly mesh with the destination sprocket. As a result, the cyclist must reduce the pedaling effort to allow the chain to properly mesh with the destination sprocket. Understandably, the resulting delay can be devastating to a high performance cyclist during a competitive event.
Another disadvantage of known high-normal rear derailleurs is that the shift lever used to operate the rear derailleur must be rotated in a direction opposite the direction of rotation of the shift lever used to shift the front derailleur to accomplish the same result. For example, if the cyclist wishes to shift to an easier gear he or she must either rotate the front derailleur shift lever downwardly or rotate the rear derailleur shift lever upwardly. This opposite motion can be confusing to people who are not very experienced with bicycle operation.